This invention relates to a plasma display apparatus and more particularly to a drive of AC refresh-type plasma display panel.
A typical example of a conventional AC refresh-type plasma display panel (PDP) to be used in the present invention includes two glass plates having electrode groups which are coated with a dielectric layer. The two glass plates are arranged in a manner which makes electrodes of respective glass plates opposed to each other. Electrodes on each glass plate intersect each other perpendicularly to form a matrix display type. The glass plates are sealed air-tightly with glass frits. Neon gas is filled in the sealed space so as to exist between the glass plates.
When the driving circuit applies a pulsed voltage to electrodes on only one glass plate while maintaining the electrodes on the other glass plate at potential zero, discharge occurs between opposed electrodes to display an image. The voltage discharged at the cell which is the most easy to discharge within the PDP is defined as the minimum unilateral discharge voltage (VDmin). The voltage discharged at the cell which is the most unlikely to discharge within the PDP is defined as the maximum unilateral discharge voltage (VDmax). If electrodes on one glass plate of the PDP have a first pulse train applied thereto with a high voltage (V0) which is higher than VDmin but lower than VDmax while the electrodes on the other glass plate have a second pulse train applied thereto with a low voltage (V1) which has a phase same as or opposite to the first pulse train, the discharge does not occur when the relation holds; VDmin&gt;.vertline.V0.vertline.-.vertline.V1.vertline. and discharge occurs when the relation holds; VDmax&lt;.vertline.V0.vertline.+.vertline.V1.vertline..
U.S. Pat. No. 4,859,910 issued on Aug. 22, 1989 discloses a new driving method for plasma display panels which results in a high level of brightness, small power consumption and a larger operating range. According to this prior art, the potential difference applied to either selected cells or non-selected cells during one scanning cycle includes a period of an address mode and a period of a hold mode. In the address mode period, a potential difference larger than VDmax is applied to discharge the selected cells while a potential difference smaller than VDmin is applied to the non-selected cells so as not to discharge them. In the hold mode period, on the other hand, the potential difference applied to both of the selected cells and non-selected cells is reduced, but the potential difference has the same amplitude such that the selected cells can continue in the discharge state while the non-selected cells require enough time to start discharge. More particularly, the address mode can be obtained by applying a pulse train of low voltage to a data electrode with the pulse train of high voltage applied to a scanning electrode. The hold mode can be obtained by applying a DC voltage to the data electrode while a high frequency voltage is applied to the scanning electrode. Further the brightness can be increased with a smaller power consumption by increasing the frequency of the hold mode period larger than that of the address mode period. The frequency during the address mode is selected to be low such as 400 KHz to 600 KHz so as to ensure the discharge thereof. In the hold mode, its frequency is selected between 1.5 MHz to 3 MHz.
In this prior art driving circuit, however, both of pulse train in the address mode and the hold mode has a duty factor of 1/2. The duty factor is defined as a ratio of a pulse width t to a pulse period T (t/T). When a display panel has a large number of pixels with high density, erronious discharge tends to occur. A discharge cell creates exciting particles such as electrons and positively charged particles. Since the mobility of electrons is several hundred times larger than that of the charged particles, the electrons diffuse at first toward an adjacent cell when a positive pulse is applied to the next scanning electrode associated with the adjacent cell along the direction of the scanning electrode. After that, the positively charged particles diffuse toward the adjacent cell along the same direction so as to compensate an electrical nutrality thereat. When these exciting particles reach to the adjacent cell, the cell becomes easy to discharge. At this moment, if the positive pulse is still applied to the scanning electrode associated with the adjacent cell, erronious discharge occurs.
It is possible to prevent such erronious discharge by increasing a frequency of the pulse train of the hold mode further so as to decrease its pulse width. However, such a high frequency operation cause a problem such as low luminescence efficiency due to the fact that a next pulse causes a new discharge before a pair of previous discharges due to a previous pulse have been accomplished.